An open Highway Addressable Remote Transducer (Highway Addressable Remote Transducer, HART) communication protocol is a communication protocol used between a smart field instrument and a control room device. An HART apparatus provides communication of relatively low bandwidth and proper response time, and a HART technology has tended to be mature at home and abroad and has become one global industrial standard of smart instruments.
The HART protocol uses a frequency shift keying (Frequency-shift keying, FSK) signal based on Bell (Bell) 202 standard, where an alternating current digital signal of an amplitude of 0.5 mA is superposed on a 4-20 mA direct current analog signal so as to perform bidirectional digital communication, a data transmission rate is 1.2 kbps, and alternating current digital signal frequencies that represent place values “0” and “1” are separately 2200 Hz and 1200 Hz. Because an average value of the FSK signal is 0, no interference is caused to the direct current analog signal when the low-level alternating current digital signal is superposed on the 4-20 mA direct current analog signal, thereby ensuring compatibility with an existing analog system.
Although the alternating current signal superposed on the direct current signal does not cause any interference to the direct current signal in a transmission process, the direct current signal is affected by the alternating current signal when it is required to extract the direct current signal from the superposed signal, so that a deviation occurs between an extracted direct current signal and an original signal. In a traditional data sampling method, dedicated data processing hardware may be used to extract the direct current signal from the superposed signal. Although a direct current signal of relatively high precision may be extracted in this method, costs increase because a dedicated hardware device is introduced. In some other traditional methods, the direct current signal also is extracted by only programming existing hardware; however, precision of most extracted direct current signals is still relatively low and cannot meet a requirement of subsequent data processing in some cases.
Therefore, it is necessary to provide a new superposed signal sampling apparatus and sampling method to solve the foregoing problem.